This invention relates to a process for purifying crude polyisocyanates.
It is known from the literature that crude isocyanate mixtures can be purified by extraction. For example, U.S. Pat. No. 3,144,474 describes a process for purifying isocyanates in which an aliphatic hydrocarbon mixture is added to a 30 to 60% solution of crude isocyanate in chlorinated aromatic hydrocarbons to precipitate out the secondary products. The purified isocyanate is recovered from the solvent mixture by distillation.
German Offenlegungsschrift No. 25 32 722 describes a process for purifying 4,4'-diphenyl methane diisocyanate (MDI). In this disclosed process, a crude mixture which contains at least 80% of 4,4'-MDI in addition to other isomers and up to 5% by weight of relatively high molecular weight fractions obtained by distillation of the isocyanate mixture obtained by the phosgenation of aniline-formaldehyde condensates is used as the starting material. Secondary products and isomers are separated from the mixture by dissolution in hydrocarbons at temperatures of up to 80.degree. C., followed by cooling. The 4,4'-MDI is obtained as end product from the hydrocarbon solution by rectification or crystallization.
German Offenlegungsschrift No. 15 43 258 describes a process for purifying crude polyisocyanates by selective extraction with two solvents of which one is a solvent and the other nonsolvent for the isocyanate mixture, in certain quantitative ratios. The purified polyisocyanate obtained as extract must be separated by distillation from the extractants.
One feature common to each of these processes is that the purified isocyanate is obtained in the form of a solution having a concentration of, in general, only 5 to 20% due to the quantity of extractant required. The solvent must be distilled off at considerable cost in terms of energy to recover solvent-free isocyanate useful for technical applications. In addition, this working up by distillation involves thermal stressing of the isocyanate which may result in the formation of secondary products.